nACTKRIOLOGIC EXAMINATION OA' SO/L. 175
amount of this soil is mixed thoroughly and the mixture
solidified upon the walls of an Ksmarch tube. The col-
onies are counted with the aid of a lens. Pliigge found
in virgin earth about 100,000 colonies in a cubic centi-
Samples of earth, like samples of water, should be
examined as soon as possible after being secured, for,
as Gunther points out, the number of bacteria changes
because of the unusual environment, exposure to increased
amounts of oxygen, etc.
The most important bacteria of the soil arc those of
tetanus and malignant edema, in addition to which, how-
ever, there are a great variety which are pathogenic for
rabbits, guinea-pigs, and mice*.
In the " Bacteriological Examination of the Soil of
Philadelphia,n Ravenel * came to the conclusion that—
1. Made soils, as commonly found, are rich in organic
matter and excessively damp through poor drainage.
2. They furnish conditions more suited to the multi-
plication of bacteria than do virgin soils, unless the latter
are contaminated by sewage or offal.
3. Made soils contain large numbers of bacteria per
gram of many different species, the deeper layers being
as rich in the number and variety of organisms as the
upper ones. After some years the number in the deeper
layers probably becomes proportionally less. Made soils
are more likely than others to contain pathogenic bacteria.
In 71 cultures that were isolated and carefully studied
by Ravenel, there were two cocci, one sarcina, and five
cladothrices; all the others were bacilli.
1 Memoirs of the National Academy of Scieuceh, Kir»st Memoir,